How to Weld Steel Pipe to Stainless Steel Pipe

The standard welding processes TIG, MIG, and stick are all capable of welding carbon steel pipe to stainless steel pipe. Standard carbon E-class welding electrodes like E70 for MIG or 7018 for stick will hold the pipe together, but the weld will be compromised by the lack of chromium and nickel in the alloy. Therefore a high-carbon stainless steel electrode must be used to seal the weld joint. 309 welding electrodes offer the correct composition to ensure that the weld has proper penetration and structural integrity.

Cut the end off of the welding wire protruding from the welding tip of the MIG welder with the wire cutters. Back spool the welding wire to remove the wire from the MIG gun and lead. Remove the spool of wire. Spool the MIG welder with the 309 welding wire.

Exchange the gas with the three part shielding gas mixture. Before attaching the three part shielding gas, lay the gas bottle on its side and roll the tank back and forth for two minutes to ensure a thorough mixing of the gas. Attach the gas gauges to the three-part gas bottle.

Turn on the MIG welder and set the heat and wire speed. Due to the higher amount of heat required to weld stainless steel, your standard heat settings for carbon steel will be inadequate. Look in your welding guide to determine an initial test setting for your heat and wire speed. Run a test pass with the welder to ensure that the settings are correct.

Cut two pieces of stainless steel pipe and two pieces of carbon steel pipe 4 inches long to use as test pieces. Mark the pipe with the permanent marker and the pipe wrap. Cut the pipe with the band saw. When cutting the stainless steel, reduce the speed and use cutting fluid to keep the band saw blade cool.

Attach the hard grinding wheel to the grinder. Place a 20-degree bevel on end of the cut test pieces of pipe with the grinder. Hold the grinder at a consistent 20 degrees to give you an even fill line. Take care when grinding the pipe. Heat builds quickly especially on stainless steel pipe. The pipe will get hot enough to produce burns in a matter of seconds.

Place one beveled end of the steel pipe against a beveled end of the stainless steel pipe. Allow a 1/16th weld gap between the two pieces of pipe. Place a tack in the weld joint to hold the pipe together. Roll the pipe and place a tack every inch around the entire circumference of the pipe. Ensure that at each tack you hold the 1/16th inch weld gap.

Weld the pipe together. Start the weld on the stainless steel pipe and pull the puddle onto the carbon steel pipe. Roll your MIG gun in a circle to flow the puddle, dropping 1/8th of an inch at each down stroke to create a nice "roll of dimes" weld. Watch the bottom of the weld to ensure proper penetration. If you find that the carbon pipe is under-cutting, reduce the heat on your MIG welder and try again. When reducing the amount of heat fails to reduce the undercut, quicken the up stroke of your circle to pull the heat away from the steel pipe.